Various methods are known for producing silicon single crystals to be used for semiconductor substrates, and examples thereof include the Czochralski method (hereinafter referred to as “the CZ method”) which is widely adopted as a rotation pulling method.
FIG. 5 is a schematic view of an essential constitution of a pulling apparatus adapted to perform a silicon single crystal growth method based on the CZ method.
The pulling apparatus has an external appearance constituted of a chamber (not shown), and crucibles are disposed at the center of the apparatus. The crucibles have a double structure which is constituted of an inner holding vessel made of quartz that has a bottomed cylindrical shape (hereinafter, simply referred to as “a quartz crucible 1a”), and an outer holding vessel made of graphite that also has a bottomed cylindrical shape, which is adapted to hold an outer surface of the quartz crucible 1a (hereinafter simply referred to as “a graphite crucible 1b”).
These crucibles are fixed to an upper end of a supporting shaft 6 so as to allow the crucibles to rotate and to move upwardly and downwardly. A resistance heating heater 2 is disposed outside the crucibles substantially concentrically, and a silicon raw material of a predetermined weight introduced into the quartz crucible is melted by the heater 2 to form a melt 3.
A pulling wire (or a pulling shaft, and hereinafter, combination of these is referred to as a “a pulling member 5”) configured to rotate about the same axis with the supporting shaft 6 at a predetermined speed in the same direction as or the opposite direction to that of the supporting shaft is disposed along a central axis of the crucible filled with the melt 3, and a seed crystal 7 is held at a lower end of the pulling member 5.
In such a pulling apparatus, the silicon raw material is introduced into the quartz crucible and is melted by the heater disposed around the crucibles in an inert gas atmosphere under reduced pressure, followed by dipping the seed crystal held at the lower end of the pulling member into an surface of the formed melt, and followed by pulling up the pulling member while rotating the crucibles and the pulling member, to grow a crystal from a lower end surface of the seed crystal.
In the CZ method, in order to eliminate dislocations inherently included in the seed crystal as well as dislocations to be brought about by thermal shock upon contact with the melt, after a necking process for once constricting a crystal growing from the lower end surface of the seed crystal into a diameter of about 3 mm, a cone portion for forming a body having a predetermined diameter (a constant diameter portion) is formed, and a silicon single crystal 4 is subsequently grown at a predetermined diameter. During it, the quartz crucible is rotated in the same direction as or the opposite direction to that of the seed crystal. Upon achievement of a targeted length of the single crystal, a tail-constricting operation of its end portion is conducted, thereby completing the growth of the single crystal.
As described above, in case of pulling of the silicon single crystal based on the CZ method, the melt obtained by melting the silicon raw material is held in the quartz crucible among the crucibles having the double structure. When the quartz crucible holds the silicon melt, a quartz crucible surface is subjected to a high-temperature of 1,500° C. or higher, and the subjected time typically comes up to several tens of hours or longer, though such a time varies depending on conditions such as a filled amount of the silicon raw material, a crystal growth rate, and the like.
Furthermore, in order to enhance productivity and a yield of the pulling of the single crystal, there has been recently developed a recharge pulling method (the RCCZ method; see “Semiconductor silicon crystal engineering”, pp. 72-73, Fumio Shimura, MARUZEN Co., Ltd., for example) configured to produce a plurality of single crystals from the identical quartz crucible. In the above-described recharge pulling method, a time, during which the quartz crucible is subjected to a silicon melt, may exceed 100 hours.
Typically, brown cristobalite called “a brownish ring” is formed at an inner wall surface of the quartz crucible during the contact of the quartz crucible with the silicon melt in a high-temperature state, and the brownish rings are gradually grown. When the brownish rings are separated from the quartz crucible during a pulling process of the single crystal, the brownish rings obstruct growth of the crystal and cause to generate dislocations of a single crystal into the crystal.
To prevent the generation of dislocations in the single crystal caused by crystallization of the inner surface of the crucible as described above, there has been disclosed a method for generating devitrification at the inner wall surface of the quartz crucible, by causing the quartz crucible to include one kind or two or more kinds of alkali metals of Na, K, and Li, and by applying a direct current voltage of a constant value in such a manner that an outer wall of the quartz crucible acts as a positive electrode, and a seed crystal (a silicon single crystal side, a pulling member side) acts as a negative electrode (see Japanese Patent Laid-open (Kokai) No. 2006-36568, for example).
However, in case of using the above-described method, since the direct current voltage is applied in such a manner that the silicon single crystal side acts as a negative electrode throughout the step of the pulling of the single crystal, there arises a problem such that high concentrations of alkali metals (particularly, Li) are taken in the crystal, and abnormal growth of an oxide film is brought about due to the Li in the crystal in a thermal oxidation treatment after wafer processing.